A good set of "closed back" headphones ought to do the trick. They'll give you the best bass response. I don't know if that Mustang amp of yours will give you true speaker emulation through the headphone jack or not, but that's the only way you'll be able to appreciate what you hear. For example, if you had a Frontman amp, it will suck when you use the headphone jack, because you're getting a straight "wide open" signal to the headphone jack. Headphones can range in price from $19 to beyond $400 and points in between. Time to do some homework and find out what works for you. Good luck.